There is a growing consensus that the best way to manage Alzheimer's Disease (AD) will be through preventive therapy. To facilitate preventive therapy, it is important to develop AD-related biomarkers that can be used to identify at risk individuals in the same way that cholesterol levels are used to identify those at risk for atherosclerotic heart disease. For this reason, we proposed in the last cycle to determine if plasma AB40 and/or AB42 might be useful biomarkers for identifying at risk individuals. In 563 normal subjects that we followed longitudinally, the plasma AB42/40 ratio was an excellent biomarker for identifying those who developed Mild Cognitive Impairment or AD in three to five years. The cumulative incidence of AD/MCI was significantly greater in subjects with an AB42/AB40 ratio in the lowest quartile as compared to those with a ratio in the highest quartile after adjusting for age and ApoE4. Subjects with an ApoE4 allele and a low (below median) AB42/40 ratio, began to develop AD/MCI at 2-3 years and, by 5 years, over 20% of the subjects in this group were affected. In contrast, only 3% of the ApoE 4 carriers with a high (above median) AB42/AB40 ratio developed AD in five years. Combining age and the AB42/AB40 ratio was also highly effective in separating subjects who developed disease from those who did not. Older subjects (age >80 years) with a low (below median) AB42/40 ratio began to develop AD/MCI at 2-3 years and, by 5 years, over 20% of the subjects in this group were affected. In contrast, less than 4% of all other subjects developed AD within five years. If these findings can be confirmed, it seems likely that the plasma AB42/AB40 ratio can become an important biomarker for developing and implementing a preventive approach to AD therapy. Our specific aims are to (1) confirm that the plasma AB42/AB40 ratio is a useful biomarker for identifying those who will develop MCI or AD in three to five years, and (2) determine if elevated AB (AB40 and/or AB42) is useful for identifying those who will develop MCI or AD in five to fifteen years. Several additional biomarkers will be evaluated in the same longitudinal series where plasma AB is analyzed. Dr. Wyss-Coray will analyze BDNF, AcrpSO (aka adiponectin), angiogenin, PDGF-BB, and MCP-1. Dr. Jack will analyze hippocampal atrophy as well as whole brain atrophy using the Boundary Shift Integral (BSI) approach. The utility of these additional biomarkers will be evaluated singly as compared to plasma Aft and jointly with plasma AB.